The barnacle nervous system has morphological and physiological features which make it an attractive preparation for the detailed analysis of the mechanisms underlying processing of visual information. In particular, the photoreceptors, their presynaptic terminals and second-order neurons are visible in the dissecting microscope, impalable with microelectrodes, and respond with graded potentials to physiological stimulation. We intend to analyze two aspects of this system: a) the control of calcium in the presynaptic terminals of the photoreceptors, and b) regional specialization and synaptic integration in the dendritic processes of the second-order cells. In addition to conventional electrophysiological and anatomical techniques we will use optical methods in these experiments: metallochromic indicator dyes will detect changes in intracellular calcium and voltage-sensitive dyes will be used in the integration studies where microelectrodes are not feasible.